Thermocouples generally known in the art are, for example, those comprising a conductor passing through an insulator tube of ceramics, a sheathed thermocouple comprising a thermocouple element passing through a tube of a heat resistant alloy, such as a stainless steel alloy, which is filled with particulates of a metal oxide such as magnesium oxide, and the like.
The thermocouple which is insulated by an insulator tube of ceramics has disadvantages such as inferior flexibility and bulkiness. When such a thermocouple is employed under a high vacuum, the insulator adsorbs a large amount of gases since the insulator is essentially porous and has a large surface area.
In the sheathed thermocouple, which is formed by a heat resistant alloy tube and a thermocouple element, on the other hand, the outer diameter is increased and the ends are hard to process.